As an expert in the field of automotive efficiency, I can provide some insights into the relationship between driving speed and fuel efficiency. The concept of fuel efficiency is often misunderstood, and it's important to understand the factors that influence it.

Firstly, it's crucial to recognize that fuel efficiency, or fuel economy, is typically measured in miles per gallon (mpg) or kilometers per liter (km/L). This metric indicates how far a vehicle can travel with a given amount of fuel. The efficiency of a vehicle is influenced by a variety of factors including engine design, vehicle weight, aerodynamics, tire pressure, and driving habits.

When it comes to driving speed, there is a certain range where a vehicle operates at its most efficient point. This is often referred to as the "economical cruising speed." For many vehicles, this speed is around 55mph (90km/h). At this speed, the engine operates within its optimal range, and the aerodynamic drag is not yet significant enough to drastically reduce fuel efficiency.

However, increasing the cruising speed beyond this point can lead to a decrease in fuel efficiency. This is primarily due to the fact that as speed increases, so does the aerodynamic drag, which is the resistance a vehicle experiences from the air it moves through. At higher speeds, the engine has to work harder to overcome this drag, which requires more fuel.

The statement that increasing your highway cruising speed from 55mph (90km/h) to 75mph (120km/h) can raise fuel consumption by as much as 20% is quite plausible. This is because the drag force increases with the square of the speed. So, when you double the speed, the drag force increases by a factor of four. This exponential increase in drag leads to a significant increase in fuel consumption.

On the other hand, driving at a slower speed than the economical cruising speed can also lead to lower fuel efficiency. This is because the engine may not be operating at its most efficient point, and there may be other inefficiencies such as increased rolling resistance and engine friction at lower speeds.

Improving your gas mileage by 10-15% by driving at 55mph rather than 65mph (104km/h) is a reasonable estimate. By maintaining a speed closer to the economical cruising speed, you can optimize the balance between engine efficiency and aerodynamic drag, resulting in better fuel economy.

It's also worth noting that modern vehicles are equipped with technologies such as variable valve timing, direct fuel injection, and turbocharging, which can improve fuel efficiency at a wider range of speeds. However, even with these advancements, the basic principles of aerodynamics and engine operation still apply.

In conclusion, while driving faster can feel exhilarating, it comes at the cost of fuel efficiency. To maximize fuel economy, it's best to drive at speeds that are close to the vehicle's economical cruising speed. This not only saves fuel but also reduces emissions and contributes to a more sustainable mode of transportation.

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Increasing your highway cruising speed from 55mph (90km/h) to 75mph (120km/h) can raise fuel consumption as much as 20%. You can improve your gas mileage 10 -C 15% by driving at 55mph rather than 65mph (104km/h).read more >>